The most important application feature that distinguishes a power circuit breaker from a molded case circuit breaker is the ability of the power circuit breaker to tolerate very high over current levels without tripping. The maximum current level that a power circuit breaker can tolerate for a short time period without internal damage is called its short time withstand current rating; generally, short time current ratings for 0.5, 1.0 and 3.0 seconds are established. The short time withstand current rating identifies the mechanical and thermal ability of the breaker to withstand overcurrents for the given period of time and is specified as a characteristic of the breaker independent of the current levels at which the trip functions are actuated. Power circuit breakers are typically used in radial distribution systems to feed a load center, motor control center or panel boards. A multiplicity of circuit breakers in these load centers then feed a variety of individual loads. To coordinate the tripping characteristics of the power circuit breaker with these downstream breakers, it is very desirable to design the mechanical characteristics of the breaker so its "withstand current" level is as high as possible, preferably equal to the available fault current from the source of supply.
If power circuit breakers are applied within the limits of their short time current rating, they are generally applied without an instantaneous trip feature. They can tolerate any available fault current for that short time and give the downstream circuit breakers an ample opportunity to clear any fault that may develop on one of the load lines. Only if the fault is located immediately downstream (with no intervening circuit breaker) should the power circuit breaker ultimately trip open. For any other fault location, the power circuit breaker should stay closed. Thus, continuity of service is preserved for all of the feeder loads that are not directly involved with the fault. This feature, where only the circuit breaker immediately upstream from any fault opens, is called "selective coordination" or "selectivity."
For many modern power circuit breakers it is common to find that the rated interrupting capacity of the power circuit breaker will exceed its short time withstand rating by a considerable margin. For example, a power circuit breaker may have an interrupting capacity rating at 480 vac of 100,000 amps, but a withstand current rating of only 65,000 amps. It is not uncommon, therefore, to find power circuit breakers applied in systems in which the available fault current can greatly exceed the rated short time withstand current rating of the breaker (but not the interrupting capacity). In such applications, the power circuit breaker must trip without intentional delay for fault currents which exceed its "withstand" capability. This is frequently called an "override" trip feature. When using this override trip feature, total selectivity is sacrificed. To minimize the chance that the power breaker will trip unnecessarily, it is important to set the override trip current setting on the power breaker as high as is possible, preferably just below its withstand current rating.
Thus, to maximize selectivity it is desirable to design the override trip setting of the power circuit breaker at or near the withstand rating. However, many power breakers in switchboards with very high available fault currents are used as feeders with frame ratings of only a few hundred amperes. In these applications, the withstand rating of the breaker (and thus, the desirable override trip setting) may be a hundred or more times the breaker frame rating. Since the current sensors in the breakers are designed for useful accuracy and power output over its "normal" current operating range, at such high overcurrent conditions they saturate magnetically and their output is totally unpredictable. Thus with conventional magnetic core current sensors, differentiating between high and very high overcurrents is not possible. In most applications a high normal current is considered to be one that is likely to be experienced intermittently under "normal" conditions such as the inrush current during motor starting; such currents are approximately ten times the normal operating range experienced while the equipment is in a steady state. Very high currents are usually associated with shorts in the circuit and currents can rise to an order of magnitude of 100 or more times the normal operating range.
A second problem that can arise when a current transformer experiences a very high overload is that the secondary current (or power) output produced can be destructive to the electronic trip unit or to the inter-connecting wiring. It is therefore common to "protect" the trip unit from such very high overcurrents by "clamping" the signal at a safe level (such as with a Zener diode), or by designing the current sensor to saturate at modest current levels, thus limiting its output. Either of these "solutions" creates an impediment to setting the override trip setting at the desired high level needed to maximize selectivity.
The most common design approaches to this problem are to (1) lower the interruption rating of the circuit breaker to equal the withstand rating so that a high-set instantaneous trip feature is not necessary, or (2) set the override trip at a very modest level (about twenty to thirty times the frame rating) and accept the loss of selectivity that results. From an application standpoint, neither of these alternatives is desirable. If the breaker interruption rating is lowered to obviate the need for an override trip function, it forgoes its application to the "high interruption" segment of the market. If the override rating is set at a low level (20 to 30 PU) consistent with the overcurrent sensing capability of the magnetic core sensors, the breaker cannot make use of its withstand capability and thus sacrifices selectivity that is otherwise inherent in the product.
There is also a segment of the market that tends to apply circuit breakers up to their interruption rating, without regard for selectivity. A portion of this market occasionally employs power circuit breakers as simple switches requiring no overcurrent protection and thus, no overcurrent trip unit. Alternatively, they may be sold to users that prefer to provide their own overcurrent protection system. Such switches are termed non-automatic circuit breakers.
If a non-automatic circuit breaker is applied in a distribution system where the available fault current level is at or below its withstand rating, it can be used without any overcurrent protection. In this instance, the manufacturer is forced to assign a lower interruption rating to the breaker which is equal to its withstand capability. It would be advantageous to be able to sell non-automatic circuit breakers with a simple, inexpensive self-contained protection device that would trip the breaker without delay at current levels above its withstand rating. With such a protection mechanism, the manufacturer could upgrade the interruption rating of the non-automatic circuit breaker to a much higher level, consistent with its ultimate interruption rating. To provide such differentiation today, manufacturers must equip the circuit breaker with current sensors and a simplified electronic trip unit at considerable expense.
Accordingly, an improved electronic trip unit and current sensor system is desired that is capable of differentiating between high and very high currents as described above and tripping on currents approaching the withstand current rating of the circuit breaker.